Onboard aircraft fire suppression systems for fires occurring in cargo compartments and engines require fire suppressants (or fire suppression agents) that are non-corrosive, volatile, electrically non-conductive, compatible with aircraft materials, have a low freezing temperature, and are non-toxic at fire suppressant concentrations. Fire suppression systems are designed to reduce the intensity of fires to a non-hazardous state.
Depending on a particular fire and the fire's environment, fire suppressants, under certain conditions, may extinguish a fire by eliminating or disabling the combustion process. A fire suppression system in an aircraft generally first knocks down a fire, and then maintains the fire suppressed for a time period sufficient for the aircraft to safely land at the nearest airport. This time period can be lengthy for certain aircraft missions, such as over-water flights.
Presently, halogenated derivatives of methane gas, such as Bromochlorodifluoromethane (CBrClF2, commonly known as Halon 1211) and Bromotrifluoromethane (CF3Br, commonly known as Halon 1301), have been found to exhibit desired fire-suppressant properties. Further, relatively small volumes of Halon 1211 or Halon 1301 are required to be delivered to a compartment to extinguish a fire in the compartment. Halon 1211 is commonly used as a Streaming agent in portable (hand-held) fire extinguishers and Halon 1301 in total flood fire suppression systems.
However, halogenated methane derivatives are deleterious to the environment. Halon 1301, for example, has long atmospheric life and slowly migrates to the stratosphere where it catalytically destroys ozone. Under international agreement known as the Montreal Protocol, Halon production ceased in developed countries on Jan. 1, 1994. Existing supplies of Halon are used in commercial aircraft under renewable “critical use exemption” granted by the regulatory agencies of governments that are signatories to the Montreal Protocol.
As a result, efforts have been made to find replacement fire suppressants as effective as Halon 1301 and Halon 1211 that are environmentally safe and that can be used with a low-weight penalty. Fire suppressants examined include halogenated hydrocarbons (commonly known as halocarbons), water-mist, inert gases and aerosols. Methods evaluated include use of a single suppressant and the use of two different agents, sequentially, to suppress fire in the cargo compartment. However, in general, the overall weight penalty can be high and the system may be complicated compared to present Halon 1301 systems. In addition, some methods may generate high concentrations of corrosive and toxic chemicals.
Some of the inert gas agent is lost to the outside during constant pressure flooding of an enclosed space in “free efflux” total flooding systems. Generally, volumes of inert gas agents required are relatively larger than the volumes of Halon 1301 to extinguish a fire. As a result, use of agents other than Halon 1301 in currently known aircraft fire suppression systems could introduce into a compartment volumes of the agent that are sufficient to over pressurize the compartment. This could result in damage of the compartment walls and loss of compartment integrity essential for operation of the total flood system.
Thus, there is an unmet need to knock down and suppress fire occurring in an enclosed space using a lower-weight suppression system that complies with regulatory requirements, is friendly to the environment, and does not pose a threat to compartment integrity.